← Back to team overview

kicad-developers team mailing list archive

Re: Rationale for rounding BIUs


On 06/25/2012 05:11 AM, jp.charras wrote:
> Le 25/06/2012 08:32, Lorenzo Marcantonio a écrit :
>> On Sun, Jun 24, 2012 at 06:23:46PM +0200, jean-pierre charras wrote:
>>> I am afraid I do not really understand your question.
>>> In float to integer conversions, rounding creates a smaller error than truncating.
>>> And truncating fval<  0 ? fval - 0.5 : fval + 0.5 is the right way to round a value that must be converted to integer BIU.
>> Right, I was asking before in other circumstances (i.e. motion control)
>> truncation or rounding is often (I can't say always, but almost)
>> replaced by floor()ing, to avoid a discontinuity issue around zero
>> (reason: the default rounding minimized biasing, flooring minimizes
>> discontinuities, more or less; I don't know the proper math terms...)
>> I generally simply avoid floating point if possible using fixed point
>> instead, if dynamic range is not needed; there is also the issue of the
>> rounding mode in use and so on (wikipedia treats everything in
>> details...).
>> In short I asked because I have simply never seen that construction (but
>> in fact maybe it's simply equivalent to floor(x+0.5) :P). Just to know
>> better the code.
> It is.
> However in debug mode, KiROUND print a warning if the rounding cannot be made.
>>> By the way, your recent issue when reading your board file in kicad nanometer version is a truncating issue,
>>> when scaling float values read in files to integer internal units.
>>> When using rounding conversion instead of truncating, it happens no more.
>> Wasn't that a precision issue caused by internal rounding of
>> printf/scanf using doubles instead of long doubles? (I would have simply
>> saved the nanos as integer...) If that was the case changing precision
>> shouldn't have changed the result!
> This is not a printf/scanf issue using doubles, this is a scaling issue.
> Here is an example ( in decimal, with one digit mantissa ) that shows can happen:
> If I use a scaling factor FILE2IU= 3, and I want to save the IU integer value = 1000
> The saved value is 333.3 (with truncation)
> after reading the file you get 333.3 * 3 = 999.9
> With truncation, new IU = 999
> With rounding, new IU = 1000
> regardless the size of the float number, i.e. the number of digits after the decimal separator.
> For this kind of issue, the fix is easy (however find the reason was tricky)
> (It was just a read function where the call to the rounding conversion was forgotten)
> And saving values using the internal values "as is" is a very bad idea.
> Values in files *must* be independent of the internal representation,
> that can be modified without breaking the file format
>> Anyway, this is my view about float to int conversion: if possible,
>> avoid it; otherwise just do whatever works (not very scientific,
>> I know...)
> If possible, yes.
> However, there is a lot of case where it is not possible, or even bad,
> mainly in trigonometric calculations:
> rotations, moving a line and keep slope, distance between items...
> Fortunately rounding errors are not always an issue, but sometimes they are.
> And remember using ints *does not* avoid truncation errors in calculations.

Maybe, but what about basic "please and thank you."  Where does that fit in?
Obviously nowhere for Lorenzo.

Follow ups